1. Field of the Invention
The invention pertains to a method of crystallizing amorphous silicon, and more particularly, to a method of fabricating crystalline silicon using an alignment key and a switching device using the crystalline silicon.
2. Description of the Related Art
Flat panel display (FPD) devices have portability and low power consumption, and they have been the subject of much recent research due to the coming of the information age. Among the various types of FPD devices, liquid crystal display (LCD) devices are widely used as monitors for notebook computers and desktop computers because of their high resolution, ability to display colors and superiority in displaying moving images.
In general, an LCD device includes two substrates disposed such that respective electrodes of the two substrates face each other. A liquid crystal layer is interposed between the respective electrodes. When a voltage is applied between the two electrodes, an electric field is generated. The electric field modulates the light transmittance of the liquid crystal layer by reorienting the liquid crystal molecules, thereby displaying images in the LCD device.
Active matrix type display devices are commonly used because of their superior display of moving images. Active matrix-type display devices include pixel regions that are disposed in matrix and a switching element, such as a thin film transistor (TFT), is formed at each pixel region. LCD devices that include TFTs using polycrystalline silicon (p-Si) have recently been widely researched and developed. In an LCD device using polycrystalline silicon, both a display region TFT and a driving circuit may be formed on one substrate. Moreover, since an additional process of connecting the TFT of the display region and the driving circuit is not necessary, the total fabrication process for the LCD device is simplified. Since the field effect mobility of polycrystalline silicon is several-hundred times as great as that of amorphous silicon, the LCD device using polycrystalline silicon has a short response time and high stability against heat and light.
Amorphous silicon may be crystallized into polycrystalline silicon. A laser annealing method, where a laser beam is irradiated onto an amorphous silicon film, finds wide use as a crystallization method. However, since the surface temperature of the irradiated amorphous silicon film reaches about 1400° C., the surface of the silicon film is apt to oxidize at its top surface. Particularly, since the laser beam irradiates several times during the laser annealing method, silicon oxide (SiO2) may be created on the top surface of the silicon film when the irradiation of the laser beam is performed under ambient air. Accordingly, the laser beam may be irradiated under a vacuum of about 10−7 to 10−6 torr. To solve the problems of the laser annealing method, a sequential lateral solidification (SLS) method using a laser beam has been suggested and researched.
The SLS method utilizes the phenomenon that grains of a silicon film grow along a direction perpendicular to a border surface of a liquid phase region and a solid phase region of the silicon film. In the SLS method, grains grow along one lateral direction by adjusting an energy density and an irradiation range of a laser beam and moving a laser beam (Robert S. Sposilli, M. A. Crowder, and James S. Im, Material Research Society Symp. Proc. Vol. 452, pages 956˜957, 1997).
FIG. 1A shows a schematic plane view of a mask used in a sequential lateral solidification method according to the related art, and FIG. 1B shows a schematic plane view of a semiconductor layer crystallized using the mask of FIG. 1A.
FIG. 1A shows a mask 10 for an SLS method that includes a slit pattern 12 having a width of several micrometers, and a laser beam having a width of several micrometers may therefore be irradiated onto a semiconductor layer. Even though not shown in FIG. 1A, a gap between adjacent slit patterns 12 may be several micrometers. For example, the slit pattern 12 may have a width of about 2 μm to about 3 μm.
In FIG. 1B, a laser beam (not shown) irradiates onto a semiconductor layer 20 of amorphous silicon through the slit pattern 12 of the mask 10 in FIG. 1A. A region 22 of the semiconductor layer 20 irradiated by the laser beam completely melts, and grains 24a and 24b grow while the melted silicon is solidified. The grains 24a and 24b laterally grow from both ends of the irradiated region 22 and stop growing at a central portion of the irradiated region 22 to form a grain boundary 28b where the grains 24a and 24b meet. Even though not shown in FIGS. 1A and 1B, the mask 10 has multiple slit patterns 12, and a crystallized portion corresponding to the mask 10 may be referred to as a unit crystallization area. The semiconductor layer 20 of amorphous silicon may fully crystallize by repeating the irradiation of the laser beam onto different regions of the semiconductor layer 20 including the irradiated region 22.
FIG. 2 shows a schematic plane view of a semiconductor layer crystallized by a sequential lateral solidification method according to the related art. In FIG. 2, a semiconductor layer of polycrystalline silicon includes multiple unit crystallization areas 30. First and second overlapping areas 40 and 50, where a laser beam is repeatedly irradiated, are created between the adjacent unit crystallization areas 30. The first overlapping area 40 runs along a vertical direction between two adjacent unit crystallization areas 30, and the second overlapping area 50 forms along a horizontal direction between two adjacent unit crystallization areas 30. Since the laser beam is irradiated onto the first and second overlapping areas 40 and 50 several times, the first and second overlapping areas 40 and 50 have non-uniform crystallization. These non-uniformly crystallized portions cause a reduction in the display quality of an LCD device, especially when the non-uniform portions are used to form a TFT of a display region in the LCD device.